One of the major benefits of moving to Google is the massively increased storage capacity for your mailbox. Coupled with Google’s fast and flexible search of your email, deleting messages to save quota space no longer makes sense. Google introduced the option to archive your mail instead of deleting it. Archiving moves the mail out of your inbox (and out of your way) until you need it later.
This new option allows users to get mail out of the way without losing it forever. But you can still delete email when you need to. For example, you want to ensure a message is destroyed for privacy or other concerns.
In the Google web client, there are both Archive and Delete buttons. The Archive button moves email out of the inbox, but you can still access it from All Mail, which will always contain your entire collection of email.
The Delete button moves email out of the inbox and into the Trash. The web interface lets you delete either an entire conversation or one message from a conversation. Deleted messages or conversations are permanently removed from Gmail 30 days after you send them to Trash.
In a desktop client such as Outlook, there is only a Delete button. If you have configured your desktop client with Google Sync or IMAP, using the Delete button will mimic the behavior of the web client’s Archive button. The message will continue to appear in the [Archived] folder (in Outlook with Google Sync) or the [Gmail]/All Mail folder (in other clients).
To force a message to be deleted, move it to the Deleted Items or Trash folders.